The consolidation process of permanent magnetic material includes sintering process and binding process with their advantages and disadvantages. Sintered magnets have good magnetic property, but complicated fabrication process and high price. Bonded magnets exhibit slightly lower magnetic property, but have advantages of easy large scale production, precise dimension, low density, stable magnetic property and multi-polarized magnetizing, leading to extensive application in electronics industry and medical industry. Currently there are four methods used for preparing bonded magnet: compression molding, injection moulding, extrusion molding and calendaring molding. There have been many researches and applications regarding compression molding and injection molding. Especially compression molding has been deeply researched and widespread applied, due to small amount of additive, higher magnetic property and simple molding method. The amount of additive is generally to the extent where a thin coating forms on the surface of every magnetic particle, and this is usually related to the structure of magnetic particle used and particle size distribution.
In preparation of bonded NdFeB permanent magnet via compression molding, the epoxy resin with an amount about 3% of magnets mass is selected as binder due to its excellent alkali resistance and low curing shrinkage rate. The epoxy resin bonded NdFeB magnets prepared by compression molding have high coercivity, but they could not used under high temperature, and their operating working temperature is limited under 110° C., due to weak temperature tolerance of epoxy resin binder (Li Fei, Current Status on the Development and Application of bonded NdFeB Magnet [J]. Rare Earth, 1999, 63-66). To increase the temperature tolerance of bonded NdFeB magnets, it has become an important issue of developing the heat-resistant binders to improve the working temperature of bonded magnets. The heat-resistant isotropic bonded NdFeB magnets with sodium silicate as principal binder and heat-resistant epoxy resin as auxiliary binder in present invention could effectively strengthen the magnets' temperature tolerance, and their working environment temperature is up to 200° C.
A Japanese patent reports a preparation method for bonded magnet component by mixing magnetic powders with sodium silicate binder. The components could work on engines and power generators under relatively high temperature. However, due to high moisture absorption, the magnet needs surface processing before use (JPH09129466, Minami Tadashi, Nakamura Katsuya, Odakane Masaaki. Manufacture of bond magnet. Japan, H01F 41/02, 1997.). If sodium silicate and epoxy resin are used together as binder, the bonded NdFeB magnet will combine the merits of the sodium silicate and epoxy resin bonded magnets, exhibiting unique advantages of temperature tolerance, reinforcing & toughening, penetration resistance & moisture absorption resistance, and corrosion resistance, etc. Sodium silicate has good heat resistance and strength to offset the shortcoming in temperature tolerance of epoxy resin and improve the strength property of magnets. Also epoxy resin permeates into sodium silicate at molecular level, and forms inter-penetrating network structure between sodium silicate and epoxy resin after cross-linking and solidifying, which greatly improves the penetration resistance and corrosion resistance of the magnets, while further reduces its moisture absorption.
The present invention uses isotropic NdFeB magnetic powders as magnetic material, sodium silicate as principal binder, and heat-resistant epoxy resin as auxiliary binder. The isotropic bonded NdFeB magnet prepared in present invention has greatly increased temperature tolerance with a working temperature of 200° C. as well as advantages of penetration resistance and corrosion resistance.